A preliminary study in 50 families in September, 1979 in Charlottesville has suggested that treatment of the fingers with a virucidal substance (2% aqueous iodine) may reduce transmission of natural colds. Only one (8%) cold developed in 13 mothers whose fingers were regularly treated with iodine during exposure to family members with colds, compared to four (29%) colds in 14 mothers whose fingers were treated with placebo and five (36%) colds in 14 mothers who used no finger treatment during similar exposures. The rates of illness per 1000 days of exposure were 25, 91, and 104 for the three groups, respectively. There were five instances in which there was an index case with a documented rhinovirus cold, and an exposed mother with known susceptibility to that virus. No illness occurred in two exposed susceptible mothers using iodine and in one using placebo. Two exposed, susceptible mothers using no treatment acquired infection with the index cases' virus. This proposal is for a similar three-year study which will enlarge the sample size of the treated and placebo groups. Finger treatment with virucidal substances may have practical application to the control of colds in the population.